[126.03] Broad Lines in the X-ray Spectrum of 4U 1626-67

We observed the 7.7 s low-mass X-ray binary pulsar 4U
1626-67 with the Chandra High Energy Transmission Grating
Spectrometer for 40 ks. This ultracompact system contains a
disk-accreting neutron star and a white dwarf companion in a
42 min binary orbit. ASCA previously observed strong X-ray
line emission around 1 keV (Angelini et al. 1995), which was
interpreted as emission predominantly from hydrogen-like Ne
X Lyman-\alpha ions accompanied by broad absorption at 1.4
keV, which suggested an unresolved blend of Ne recombination
and H- and He-like Mg absorption edges. Blends with O and Fe
line complexes seemed highly probable leading to an inferred
overabundance of x6, x2, and x2 relative to solar for Ne, O,
and Mg, respectively. Chandra spectra fully resolves these
features. We confirm the existence of the Ne X
Lyman-\alpha line, the recombination edge as well as the
O-K emission. However, the observed lines, predominately
from He- and H-like Ne and O and Ne-like Fe ions, are
broadened, indicating very high velocities of 5000-6000
km/s. We discuss various possibilities for the origin of
these high velocity lines in the context of the geometry and
accretion mechanisms of the system. The high velocities
indicate that these lines may originate close to the
accretion stream onto the neutron star rather than from
within the accretion disk.

This work was funded in part by contract SAO SV1-61010 and
NASA contract NAS8-39073.